This invention is related to my copending application entitled PIGMENT ENCAPSULATED LATEX AQUEOUS COLORANT DISPERSIONS, Ser. No. 06/842,609, now U.S. Pat. No. 4,665,107 filed concurrently herewith.
The invention relates to a process for producing particles of polymer encapsulated solid electrophoretic material, below 2 microns in size, wherein a polymer matrix completely surrounds an active ingredient and the resulting particles are configured to have a buoyancy which is effectively neutral with respect to an organic, electrically acceptable conductive carrier fluid.
The active ingredient preferably comprises a pigment, with a primary particle size in the submicron range. A preferred pigment is titanium dioxide (TiO.sub.2) in view of its very high contrast ratio and covering power. However, the specific density of titanium oxide compounds is very high, in the range 3.8 to 4.1 gram/c.c., and there is no known electrophoretic display carrier fluid close to that density.
A preferred carrier fluid is tetrachloroethylene, which has a density of 1.6 gram/c.c. A typical electrophoretic display will comprise two plates separated by a space on the order of 50 to 100 microns. The particles must float with substantially neutral buoyancy, in the carrier fluid between the plates, and must respond to an electric field, without unacceptable charge retention or migration. A polymer coating candidate according to the invention proposed herein must not only create particles less than 2 microns in size with an acceptable density, to avoid settling, but the polymer coating must be non-reactive with the carrier fluid and capable of '' resisting any tendency to agglomerate with adjacent particles, to avoid coalescent forming at the electrodes.
Since the polymer/electrophoretic carrier fluid interaction is a critical condition, it is valuable to be able substantially to rely uponknown polymer starting material characteristics, when designing a particle electrophoretic display system. The present invention does not chemically alter the polymer structure in the process of coating electrophoretic pigment particles, and since the polymer encapsulates the pigment surface, the pigment surface morphology becomes that of the polymer. Hence, the surface charge of the electrophoretic encapsulated pigment carrier fluid interface can be rendered substantially constant, for different pigments with equivalent primary particle sizes, and equivalent susceptibility to being fully dispersed in the same polymer/solvent carrier fluid system.
The present process invention categorically is not an emulsion polymerization, since the primary particles are not dispersed in a monomer or any mixture of monomers, and no polymerization conditions need to be satisfied as part of an emulsification step. A pertinent prior art technique for producing colloidal size, hydrophobic polymer particulates surrounding discrete particles of inorganic material by emsulsification polymerization is SOLC nee Hajna, U.S. Pat. No. 4,421,660. SOLC illustrates one technique to surround a discrete primary particle such as titanium dioxide with a polymer matrix, but it is not apparent that possible emulsion polymerization candidates will meet the requirement of an electrophoretic display application. The present invention categorically avoids need critically to adjust conditions of emulsification so as to induce polymerization around a core particle, although the present invention critically requires an initial polymer solvent system that must be dispersible in a fluid such as water, and then a complete solvent removal using an energy addition step that also will not create agglomerations or phase separation.